Obsessive-compulsive disorder is an uncommon syndrome about which little is known. The purpose of this project is to collect basic data about the phenomenology, the psychobiology, and the treatment of this disorder. Results with several biologic variables which have previously been described as abnormal in patients with depression suggest that obsessive-compulsive disorder patients share some of the psychobiologic features of affective illness. These variables include dexamethasone suppression, sleep physiology, and neuroendocrine response to clonidine. Furthermore, the tricyclic antidepressant, clomipramine, has modest but consistent beneficial effects in obsessive-compulsive patients with or without secondary depression.